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Amplifier

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john89

New Member
Hi

I have a JFET transistor, and I want to determine the values of the resistors R_3, R_4, R_S and R_D, so it has an appropriate operating point.

I have found R_S, R_D and a relationship for R_3 and R_4

R_4/(R_3 + R_4) = 1/6

I also know that

U_ut/U_in ≥ 0.995

I want to find another value or relationship for R_3 and R_4, so I can solve it.

http://img216.imageshack.us/img216/5749/40833320.jpg

U_GS = Voltage Gate Source
 

RCinFLA

Well-Known Member
Do you really intend for source and drain to be tied together?

If so it is just a Vgs voltage clamp, not an amplifier.
 
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john89

New Member
sorry, there shouldn't be a connection there.

However Ive started to think whether its better to use the R3 R4 relationship and just choose for ex R4 to 2Mohm and then solve, its supposed to be high resistans.
 
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RCinFLA

Well-Known Member
Looks like a source follower. Rd can be zero ohms.

You usually want source to be near mid-supply for max output swing. Actual best swing is when source is supply/2 minus half Vgs. Gate will be half supply plus half Vgs. The value of Rs will set the current through device. Add Vgs to Vs to set the R3/R4 divider. JFET will have very small, nanoamps, of gate current so don't have to worry too much about gate bias current.
 
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